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<a name="safe_numerics.tutorial.5"></a>Mixing Data Types Can Create Subtle Errors</h3></div></div></div>
<p>C++ contains signed and unsigned integer types. In spite of their
    names, they function differently which often produces surprising results
    for some operands. Program errors from this behavior can be exceedingly
    difficult to find. This has lead to recommendations of various ad hoc
    "rules" to avoid these problems. It's not always easy to apply these
    "rules" to existing code without creating even more bugs. Here is a
    typical example of this problem:</p>
<pre class="programlisting"><span class="comment">//  Copyright (c) 2018 Robert Ramey</span>
<span class="comment">//</span>
<span class="comment">// Distributed under the Boost Software License, Version 1.0. (See</span>
<span class="comment">// accompanying file LICENSE_1_0.txt or copy at</span>
<span class="comment">// http://www.boost.org/LICENSE_1_0.txt)</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">iostream</span><span class="special">&gt;</span>
<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">cstdint</span><span class="special">&gt;</span>

<span class="preprocessor">#include</span> <span class="special">&lt;</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">safe_numerics</span><span class="special">/</span><span class="identifier">safe_integer</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">&gt;</span>

<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">std</span><span class="special">;</span>
<span class="keyword">using</span> <span class="keyword">namespace</span> <span class="identifier">boost</span><span class="special">::</span><span class="identifier">safe_numerics</span><span class="special">;</span>

<span class="keyword">void</span> <span class="identifier">f</span><span class="special">(</span><span class="keyword">const</span> <span class="keyword">unsigned</span> <span class="keyword">int</span> <span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span> <span class="keyword">const</span> <span class="identifier">int8_t</span> <span class="special">&amp;</span> <span class="identifier">y</span><span class="special">)</span><span class="special">{</span>
    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">x</span> <span class="special">*</span> <span class="identifier">y</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>
<span class="keyword">void</span> <span class="identifier">safe_f</span><span class="special">(</span>
    <span class="keyword">const</span> <span class="identifier">safe</span><span class="special">&lt;</span><span class="keyword">unsigned</span> <span class="keyword">int</span><span class="special">&gt;</span> <span class="special">&amp;</span> <span class="identifier">x</span><span class="special">,</span>
    <span class="keyword">const</span> <span class="identifier">safe</span><span class="special">&lt;</span><span class="identifier">int8_t</span><span class="special">&gt;</span> <span class="special">&amp;</span> <span class="identifier">y</span>
<span class="special">)</span><span class="special">{</span>
    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="identifier">x</span> <span class="special">*</span> <span class="identifier">y</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
<span class="special">}</span>

<span class="keyword">int</span> <span class="identifier">main</span><span class="special">(</span><span class="special">)</span><span class="special">{</span>
    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"example 4: "</span><span class="special">;</span>
    <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"mixing types produces surprising results"</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span>
    <span class="keyword">try</span> <span class="special">{</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Not using safe numerics"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
        <span class="comment">// problem: mixing types produces surprising results.</span>
        <span class="identifier">f</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="number">100</span><span class="special">)</span><span class="special">;</span>  <span class="comment">// works as expected</span>
        <span class="identifier">f</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="special">-</span><span class="number">100</span><span class="special">)</span><span class="special">;</span> <span class="comment">// wrong result - unnoticed</span>
        <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"error NOT detected!"</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span><span class="special">;</span>
    <span class="special">}</span>
    <span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span><span class="special">{</span>
        <span class="comment">// never arrive here</span>
        <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"error detected:"</span> <span class="special">&lt;&lt;</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">(</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span><span class="special">;</span>
    <span class="special">}</span>
    <span class="keyword">try</span> <span class="special">{</span>
        <span class="comment">// solution: use safe types</span>
        <span class="identifier">std</span><span class="special">::</span><span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"Using safe numerics"</span> <span class="special">&lt;&lt;</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">endl</span><span class="special">;</span>
        <span class="identifier">safe_f</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="number">100</span><span class="special">)</span><span class="special">;</span>  <span class="comment">// works as expected</span>
        <span class="identifier">safe_f</span><span class="special">(</span><span class="number">100</span><span class="special">,</span> <span class="special">-</span><span class="number">100</span><span class="special">)</span><span class="special">;</span> <span class="comment">// throw error</span>
        <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"error NOT detected!"</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span><span class="special">;</span>
    <span class="special">}</span>
    <span class="keyword">catch</span><span class="special">(</span><span class="keyword">const</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">exception</span> <span class="special">&amp;</span> <span class="identifier">e</span><span class="special">)</span><span class="special">{</span>
        <span class="identifier">cout</span> <span class="special">&lt;&lt;</span> <span class="string">"error detected:"</span> <span class="special">&lt;&lt;</span> <span class="identifier">e</span><span class="special">.</span><span class="identifier">what</span><span class="special">(</span><span class="special">)</span> <span class="special">&lt;&lt;</span> <span class="identifier">endl</span><span class="special">;</span><span class="special">;</span>
    <span class="special">}</span>
    <span class="keyword">return</span> <span class="number">0</span><span class="special">;</span>
<span class="special">}</span>

</pre>
<p>Here
    is the output of the above program:</p>
<pre class="screen">example 4: mixing types produces surprising results
Not using safe numerics
10000
4294957296
error NOT detected!
Using safe numerics
10000
error detected!converted negative value to unsigned: domain error
</pre>
<p>This solution is simple, just replace instances of <code class="computeroutput">int</code>
    with <code class="computeroutput">safe&lt;int&gt;</code>.</p>
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